The invention relates to a tracking YIG-tuned filter and mixer, particularly adaptable for use as a preselector in a spectrum analyzer.
A spectrum analyzer is a scanning receiver that displays a plot of signals and their bandwidths over a specific frequency band. To cover a broad frequency range, e.g., DC to 22 GHz, the input signal is divided into high frequency and low frequency portions. The invention is concerned with the initial processing of the high frequency portion and with switching the input signal between the high and low frequency sections of the instrument.
FIG. 1 shows a schematic diagram illustrating the initial signal processing in a conventional spectrum analyzer. A radio frequency (RF) input signal at terminal 11 is coupled through a mechanically actuated microwave relay switch 13 to the low frequency signal processing section on line 15 or to the high frequency signal processing section on line 17. Low frequency band signals, with a frequency under 3 GHz, are applied to the low frequency analysis circuits. Microwave band signals, with a frequency between 2.7 and 22 GHz are passed through a tunable bandpass filter 19, then downconverted by harmonic mixer 21. Harmonic mixer 21 combines the RF input signal with a signal from a local oscillator (LO) or a harmonic of the signal from the LO to produce an intermediate frequency (IF) signal at a frequency suitable for processing by the spectrum analyzer. A spectrum analysis measurement is performed on the RF input signal by sweeping the LO frequency over the frequency range of interest while a set IF frequency is monitored.
The graph in FIG. 2 illustrates the result of the downconversion by mixer 21, showing the relationship between the LO, RF and IF frequencies. In FIG. 2, the vertical axis represents signal power and the horizontal axis represents signal frequency. IF signal 25 has a frequency equal to the difference between the LO signal (or harmonic) 29 and RF signal 27, so that the RF signal is measured by monitoring a set IF frequency, below the LO frequency, at f.sub.RF =(n)f.sub.LO -f.sub.IF. However, an image RF signal above the LO frequency, at f'.sub.RF =(n)f.sub.LO +f.sub.IF, will also produce a signal at the monitored IF frequency. To resolve this ambiguity, filter 19 acts as a tunable bandpass filter over a frequency range including f.sub.RF, as shown by the broken line curve 24, thereby attenuating any image signal 31 at f'.sub.RF. Thus, the passband of filter 19 must track the sweeping LO signal, with the center frequency of the passband separated from the LO frequency (or harmonic) by the IF frequency.
The prior art circuit shown in FIG. 1 has several drawbacks. Mechanically actuated microwave relay switches are slow and become unreliable with long term use. Accurate tunable high frequency filters are difficult to build.
Some contemporary YIG-tuned structures use PIN diode switches. The PIN diode switches solve many of the problems caused by the mechanical switches, but to date these circuits have been limited to operation above 10 MHz.
Yig tuned resonator filters comprise a YIG sphere suspended between two orthogonal half loop conductors. The YIG material exhibits ferrimagnetic resonance. In the presence of an external DC magnetic field, the dipoles in the YIG sphere align with the magnetic field, producing a strong net magnetization M. An RF signal applied to the input half loop produces an alternating magnetic field perpendicular to the DC magnetic field. The dipoles precess around the applied DC magnetic field at the frequency of the RF signal if the RF frequency is close to the resonance frequency of the dipoles. The resonance frequency for a spherical YIG resonator is: EQU f.sub.r =.gamma. (H.sub.o .+-.H.sub.a)
where,H.sub.o is the strength of the applied DC field in oersteds, H.sub.a is the internal anisotropy field within the YIG material, and .gamma.is the gyromagnetic ratio (2.8 MHz/oersted).
If an RF signal at or near f.sub.r is applied to the input loop, the precessing dipoles create a circularly polarized magnetic field, rotating at the RF frequency, in a plane perpendicular to the externally applied magnetic field. This rotating field couples to the other conductor loop, inducing an RF signal in the loop that, at the resonance frequency f.sub.r, is phase shifted 90 degrees from the input RF signal.
Because the resonance bandwidth can be made fairly narrow, the YIG resonator makes an excellent filter at RF frequencies, tunable by varying the strength of the applied DC magnetic field.
However, it has been difficult to achieve accurate tuning in YIG-tuned filters because of the nonlinearity, hysteresis and eddy current delay in the magnetic tuning elements. If the peak of the filter passband is not centered at the RF frequency being measured, the amplitude of the measured signal is attenuated, as shown by broken line curve 24', making the measurement inaccurate.
One approach to maintaining the YIG filter at the proper tracking frequency, known as peaking the filter, involves dithering the magnetic tuning field while peak detecting the resulting IF output signal. Although this method can be effective in eliminating amplitude inaccuracy, it is time consuming.
A second approach involves adding an extra YIG filter detector in a small offsetting magnetic field. By sending a known signal to this detector and dithering the offset field, the magnetic structure can be tuned. This method, however, has a severe drawback. The dithering offset field can leak to the other YIG tuned resonators nearby, adding spurious signals to the RF signals coupled through the YIGs.